Not applicable.
The invention concerns the short circulation of a paper or board machine and in particular a simplified process arrangement for the short circulation.
In paper and board machines, the stock is fed out of a storage tank into a metering tank and from said tank further into a mixing tank, in which the stock components are mixed with each other. The mixed stock is passed into a machine tank, from which there is an overflow back into the mixing tank. The stock, whose consistency is about 3%, is fed from the machine tank into the wire pit of the short circulation, in which wire pit the high-consistency stock is diluted to a headbox consistency of about 1%. The fibres and fillers that are used as the raw-material for the paper or board are introduced on the wire through the headbox as carried by water, and the filtrate which has passed through the wire, which contains an abundance of fibrous matter and fillers, is returned as a dilution for the high-consistency stock arriving from the machine tank through the headbox back onto the wire. This flow loop is called the short circulation of the paper or board machine. Contaminations that have entered in the short circulation along with the high-consistency stock or otherwise are removed before the headbox by means of cleaning devices employed in the short circulation, which devices are vortex cleaners, screens and machine screens.
Ever stricter requirements concerning the protection of environment, in particular in respect of emissions and in respect of consumption of raw-materials and commodities and, on the other hand, an attempt to achieve better production efficiency and to minimize disturbance in the production have resulted in ever more closed systems and in a more closed short circulation and in maximally efficient recirculation of raw-materials and, on the other hand, in an ever higher wire retention level, in which case it is necessary to use a larger quantity of retention agents. The short circulations that are used currently in paper and board machines have rather complicated constructions, and the various operations in the process require apparatuses of their own, in which case a physically large process space is required and, on the other hand, the volumes in the process itself are also larger. Owing to the large process volume, the times taken by changes of grade are relatively long. The numerous expensive apparatuses require an abundance of space, and the maintenance, servicing and upkeep of separate apparatuses cause expenses. A conventional short circulation requires a number of pumps, screens and deaerators which have been dimensioned for the whole flow volume of the headbox. In the case of a dilution headbox, a double circulation is also required for the dilution water. In such a case, the first dilution takes place in the wire pit, and the second dilution in the headbox. In the future, when the running speeds of paper and board machines become higher than 2000 meters per minute, the pumping volumes become so large that the present-day systems are no longer adequate.
In the FI Patent 88,415, a process arrangement is described for production of the stock for the headbox of a paper machine in the short circulation. In the arrangement that is suggested, fresh stock is not mixed into the circulation water passing into the deaeration tank. In the arrangement, a combination wire pit is used, which has been divided into two separate compartments to constitute two jointly operative tanks. The first tank has been arranged as a feed tank for deaeration, and the second tank as a dilution tank for the headbox stock, into which tank fresh stock is fed. By means of this process arrangement, attempts have been made mainly to eliminate the essential problems caused by variations of consistency and pressure in the headbox.
The process arrangement in accordance with the present invention for the short circulation in a paper or board machine has a metering system for the stock or stocks, pumps, a headbox, and a wire part as well as a pipe system interconnecting the devices, with means of regulation, and in which arrangement a high-consistency stock is fed into the headbox, and the white water recovered from the wire part is fed by means of a diluting pump into the headbox so that diluting of the stock suspension is carried out in the headbox.
The object of the present invention is to provide a process arrangement for the short circulation in a paper or board machine, by means of which arrangement it is possible to solve and substantially to reduce the problems involved in the prior art.
It is a further object of the present invention to provide a simplified process arrangement for the short circulation, by means of which arrangement it is possible to reduce the process volumes and to integrate a number of operations with each other and to obtain economies and a process that loads the environment to a lower extent.
In the following, some preferred embodiments of the invention will be illustrated with reference to the figures in the accompanying drawings, the invention being, however, not supposed to be confined to the details of said illustrations.